1. Field of the Invention
This invention relates to methods for attaching solder balls to a substrate.
2. Description of the Related Art
In assembly of components including semiconductor chips, it is often necessary to attach conductors on one surface to conductors on another surface with solder balls. For example, the manufacture of a xe2x80x9cflip chipxe2x80x9d involves placing solder balls on the bond pads of a semiconductor chip and attaching the solder balls to corresponding conductive traces on a substrate, thereby electrically connecting the chip to the substrate.
A conventional technique for creating a flip chip is illustrated in FIGS. 1a-1b. FIG. 1a is an overhead view of an unbonded flip chip with a die 100 having an active circuit surface 102 on which are arranged a plurality of solder balls 104, which typically are an alloy comprising lead and tin. FIG. 1b is an overhead view of a substrate 108 to which the die 100 will be attached. The substrate comprises a plurality of bond pads 106, often comprising copper with a thin top layer of nickel/gold, each of a plurality of traces 118, typically copper, connected to a corresponding one of the plurality of bond pads 106. Frequently, substrates are produced by vendors that ship the substrates to whomever performs final assembly of the package. To protect the traces 118 from oxidizing and from mechanical damage during the shipment process, substrate vendors often apply an organic solderability preserve (xe2x80x9cOSPxe2x80x9d) to the traces 118.
Prior to bonding the die 100 to the substrate 108, solder flux is applied to the plurality of solder balls 104 or the plurality of bond pads 106 on the substrate 108. Typical flux materials include low-solids, no-clean fluxes such as TAC-10 (produced by Indium Corp.) and Kester 9601 (produced by Kester Corporation). The flux serves primarily to aid the flow of the solder, such that the plurality of solder balls 104 make good contact with the plurality of bond pads 106. The flux may be applied in a variety of ways, including brushing or spraying, or dipping the die 100 into a thin film, thereby coating the plurality of solder balls 104 with flux.
As shown in FIG. 1c, the substrate 108 is overlaid with a solder mask 110. The solder mask 110 is typically 35 um thick and often comprises a dielectric material. As shown, the solder mask 110 has a plurality of holes 112 therein, each of the plurality of holes 112 corresponding to one of the plurality of solder balls 104. Each of the plurality of holes 112 exposes a corresponding one of the plurality of bond pads 106; the exposed bond pads contact the plurality of solder balls 104 when the die 100 and substrate 108 are moved toward each other. The solder mask 110 keeps the solder in the area of each of the bond pads 106 and thus prevents the solder from flowing onto the plurality of traces 118. Also, in the area outside the die placement area, the solder mask 110 provides mechanical protection and surface insulation resistance to the plurality of traces 118.
FIG. 1d is a cross sectional side view of an assembly 114 comprising the substrate 108 and the die 100 after the two have been brought together. The assembly 114 is heated, causing the plurality of solder balls 104 to reflow and thus to mechanically and electrically couple a pad on the die 100 to a corresponding one of the plurality of bond pads 106. During the reflow process, the solder mask 110 prevents the solder from flowing onto the plurality of traces 118. The assembly 114 is heated within a temperature range for a certain time to activate the flux (xe2x80x9csoak timexe2x80x9d). The temperature is then increased to cause the solder to reflow. After cooling, underfill is then dispensed into a gap 116 (shown in FIG. 1d) between the die 100 and the substrate 108. Since the solder mask 110 is typically 35 um thick, it reduces the gap 116 for underfill by about 15 um to 20 um.
This reduction in the gap between a chip and a substrate increases the difficulty of dispensing the underfill; specifically, a smaller gap impedes the flow of underfill between the chip and the substrate and thereby reduces the adhesion of the underfill. Reduced adhesion, in turn, results in decreased reliability of the attachment of die to substrate under conditions of stress, such as temperature cycling and moisture preconditioning. Solder masks have other drawbacks. For example, dimensional tolerances of the solder mask openings for the pads on the substrate can limit the density (pitch) between solder balls due to substrate manufacturing defects and defects involved in the assembly of the flip chip. These defects lower manufacturing yields and increase the cost of the substrate and the cost of flip chip assembly.
Therefore, it would be desirable to control solder spread by means other than a solder mask.
The present invention satisfies the above need to control solder spread on a substrate with bond pads by means other than a solder mask. According to one aspect of the present invention, a solder mask is placed on a substrate but this solder mask will not be used to control solder spread but merely helps to protect traces that are distant from the bond pads. The solder mask has an opening that is preferably greater than the area of a die to be attached; this opening exposes both the bond pads and at least portions of traces proximate to the bond pads. Since the traces proximate to the bond pads are not protected by a solder mask, the present invention employs at least one of two methods to control the flow of solder.
According to one of the methods, an appropriate combination of a flux and an OSP is selected along with particular process parameters (e.g. the shape of the solder reflow profile) to control solder spread. According to the other of the methods, the portions of the traces that are proximate to the bond pads are oxidized, thereby preventing solder from flowing onto these portions of the traces during the solder reflow process.